Winkworth boards

Has anyone experienced Winkworths “to let”or any estate agents signs going up after the property has been let? Seems pointless yet really intrusive and damaging. Heard a massive bang outside this morning and noticed a sign go up. On further inspection it was just one nail but right into the brickwork, not tied to anything as expected. I rang Winkworth to ask if the flat was to let again and they said no that it went over one month ago. They said it just went up for advertising purposes. Can they do this? How long can it say up for? Who removes it and who repairs the damage to the brickwork?